1. Field of the Invention
The present invention relates to a fixing device to fix a toner image onto a transfer material which is developed from a latent image bearing member. The present invention also relates to an image forming apparatus using the fixing device and a process cartridge detachably attached to the image forming apparatus.
2. Discussion of the Background
An electrophotographic developing device is typically used for image forming methods such as electrophotography, electrostatic recording and electrostatic printing. The image forming methods typically include the following processes:    (1) an electrostatic latent image formed on an image bearing member, such as a photoreceptor or a dielectric material, is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (2) the toner image is transferred on a receiving material, such as a receiving paper, optionally via an intermediate transfer medium (transfer process); and    (3) the toner image is fixed on the receiving material upon application of heat and/or pressure, or the like (fixing process).
As one of the structures of such fixing devices, there is a heat roller fixing system. Such fixing systems use a heat roller having a built-in heat source and a pressing roller facing the heat roller. The heat roller and the pressing roller form a fixing nip portion.
In this heat roller fixing system, the roller used is typically made of a material having a relatively large heat capacity. Thus, it takes a long time before the roller is heated to a determined temperature (this time is hereinafter referred to as a rise-up time).
To shorten the rise-up time, published unexamined Japanese patent application (hereinafter referred to as JOP) No. 11-282307 describes a fixing device using a belt which is stretched over rollers and which stores a heating member configured to contact and heat toner images. In addition, a fixing device having a belt stretched over rollers (hereinafter referred to as belt fixing device) has been widely used in terms of energy saving and reduction in size. This is because belts have structural and characteristic advantages over rollers. One of such advantages is that a belt has relatively small volume comparative with that of a roller. Another such advantage is that thin belts can have a low heat capacity. Therefore the rise-up time can be shortened. Yet another advantage of using a belt is flexibility. Thus, when a belt is used as a heating member, it is possible to widen the contact area of the belt and the pressing member and the heat transfer area.
FIG. 1 is a diagram illustrating such a belt fixing system 50. Instead of using a heat roller, a belt 52 stretched over two rollers 53 and 54 is used. A pressing roller 51 faces the roller 54. The roller 53 contains a heat source for heating the belt 52 from the rear surface thereof. In addition, the pressing roller 51 has another heat source for heating the belt 52 from the front surface thereof.
At the time of fixing, the toner is fused by heat and pressure. Then some of the fused toner particles may be transferred-to the heating member (i.e., “hot offset”). In attempts to avoid this hot offset, for example, an offset prevention layer is formed on the heating member by coating a fluorine resin, etc. on the surface of the heating member. Alternatively, a material having a good release property such as silicone oils is applied to the surface of the heating member.
With regard to toner offsets to a heating member, other than the hot offset, there is another offset referred to as “electrostatic offset.” One phenomenon caused by such offset is that unfixed toner particles before fusing are attracted to the heating member.
This electrostatic offset happens when the toner particles weakly attached to a transfer material by its viscosity and electrostatic force reach the fixing nip portion formed by the heating member and the pressing member. At this point, some of the toner particles may be transferred onto the heating member and fused by the heating member when heat is applied to the transferred toner particles. Further, when the transferred toner particles contact the transfer material again while the transferred toner particles are still in a fusion state, the toner particles may be transferred thereto, resulting in contamination of the transfer material.
One of the causes of the electrostatic offset is triboelectric charging of fixing members, i.e., heating members and pressing members. That is, due to the difference in the processing precision levels between a heating member and a pressing member, the pressing member and the heating member may move at different speeds and then friction between both members may occur. When a transfer material moves through the fixing nip portion in such a state, triboelectic charging tends to occur since the heating member, the pressing member and the transfer material move at different speeds. Therefore, the heating member attracts the toners carried on the transfer material by electrostatic force caused by the triboelectric charging or by van der Waals force. In addition, the causes of triboelectric charging of the heating member are not only the difference in the processing precision levels. If the heating member thermally expands, its linear speed changes and triboelectric charging occurs.
To prevent the toner particle affection caused by the electrostatic offset ascribable to this triboelectric charging of the fixing members, it is known that a cleaner is provided to clean the surface of the fixing belt after the surface passes through the fixing nip portion. However, a problem occurs to even a fixing device having a cleaner when small sized recording media such as post cards are transferred in a row. That is, the temperature of the surface of the fixing belt which such recording media do not contact rises high relative to that of the surface of the fixing belt which the recording media contact. This relatively high temperature is high enough to fuse the toner particles collected by and located on the cleaner and the fused toner particles are attached to the fixing belt, resulting in contamination of recording media. In addition, it is inevitable that a fixing device having a cleaner is complicated and increased in size. In attempting to avoid this electrostatic offset, JOP No. 2002-40856 describes a fixing device using a heating member without such a cleaner. In the fixing device, the heating member such as heat rollers having a releasing layer on its surface is provided so as to contact a toner image. The releasing layer has a determined surface resistance and volume resistance (i.e., not greater than 1×108 Ω and not greater than 1×1013 Ω·cm, respectively). But the surface resistivity determined above is with regard to the fixing roller (i.e., heat roller) which contacts unfixed toners on a recording medium. The surface resistivity of the pressing roller which contacts the other side of the recording medium is not discussed.
JOP No. 2003-76190 describes a fixing device including a pressing roller having a predetermined surface resistivity. The pressing roller has a surface resistance not greater than 1×106 Ω when 500 V is applied. However, this fixing device aims to prevent electrostatic offset occurring at the time of rear end detachment of a recording medium. In addition, it is preferred electrostatic offset can be prevented without applying a bias to the pressing roller.
On the other hand, to satisfy a recent increasing demand for quality images, developers, especially toners, have been targeted for improvement. Conventional toners prepared by kneading and pulverizing have irregular forms and therefore are disadvantageous with regard to particle diameter distribution, fluidity and transferability. Further, such irregular form toners have prongs to which charges are easily concentrated, resulting in deterioration of charge ability of the toner. Instead of using this type of toner, for example, JOP No. 2002-351143 describes toners prepared by polymerization methods free from pulverization process.
As discussed above, triboelectric charging causing the electrostatic offset is mainly due to the difference in linear speeds among the members used for fixing and the transfer material, however currently it is practically impossible to eliminate the speed difference. Therefore, to address this electrostatic offset problem, it is important to provide a heating member having a structure such that the toner particles are prevented from adhering to the heating member contacting the toner particles, as described in JOP No. 2002-40856 described above.
However, the transfer of toner to a heating member is caused by not only van der Waals forces or the electrostatic force caused by triboelectric charging, but also by a repulsive force generated when the charge polarity of some of the toner particles and a pressing member is the same.
When a pressing member and toner particles on a transfer material have the same charge polarity, the toner particles may be also transferred to a heating member. That is, the toner particles charged with the same polarity as that of the pressing member receive a repulsive force from the pressing member and thereby fly and may attach to the heating member.
The toners carried on the transfer material may have the same polarity as that of the pressing member. This is caused by an AC bias which may be applied at the time of image transfer. In this case, the toner includes reversely charged toner particles. When such reversely charged toner particles face the pressing member, the reversely charged toner particles and the pressing member receive a repulsive force from each other.
As described above, JOP No. 2003-76190 describes a fixing device using a pressing member having a predetermined surface resistivity. This structure has a purpose of preventing uneven surface potential at the heating member when a bias is applied between the heating member and the pressing member. This uneven surface potential at the heating member is caused by detachment discharge generated when the rear end of a transfer material detaches from the fixing member. Therefore, this fixing device using a pressing member having a predetermined surface resistivity can prevent toner attraction to a heating member caused by repulsion between the heating members and the toner particles ascribable to uneven surface potential at the surface of the heating member. Thus, the toner attraction caused by the triboelectric charging mentioned above resulting from the difference in linear speeds of the heating member, the pressing member and the transfer material cannot be prevented by the fixing device using a pressing member having a predetermined surface resistivity.
In attempting to fulfill a need for quality images which has recently been increased, polymerized toners are often used instead of the toners prepared by conventional pulverization process to improve size distribution and fluidity. However, polymerized toners tend to be affected by environmental factors such as humidity. Thus, the repulsion force between the pressing member and the toners may increase depending on charge quantity of the toner which widely changes depending on environmental conditions.
Because of these reasons, a need exists for a fixing device capable of reducing the amount of toner offset to produce quality images by using polymerized toners.